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I'm looking for a highly accurate phase detector for my capacitive sensing application. In the application, the capacitance is in the range of 1 ~ 10 pF. I've been able to get the capacitance via analog device's AD7745. However, I was requested to have the dissipation factor measured as well. I've been trying to seek a chip to do the job, unfortunately not yet. I've came up with the idea of using a phase detector, with the information of which, I'll be able to calculate an approximate value of dissipation factor myself later on. So my question is, is there such a phase detector that can detect the phase difference between the voltage and current across the capacitive sensor with a very high accuracy? with accuracy I mean: the capacitor is a nearly ideal capacitor with dissipation factor of less than 0.001, so the most interesting part of phase difference is between 89 ~ 90 degrees...

The interested frequency range is DC ~ 1Mhz.

thanks !

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  • \$\begingroup\$ It may not work at low frequencies because of the amount of current might be difficult to generate but at high frequencies, knowing the voltage across the capacitor and measuring the temperature rise might give a decent approximation to losses. \$\endgroup\$
    – Andy aka
    Jul 3, 2013 at 21:30
  • \$\begingroup\$ hi Andy~ can you tell me a chip in question ? and from above what frequency it will be rather accurate? \$\endgroup\$
    – user25907
    Jul 3, 2013 at 21:48

2 Answers 2

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You can use the Analog Devices AD8302 phase detector, you just need to tune it up to read between 89-90 degrees thus a sensitive and calibrated micro-voltmeter to give you some good measurement resolution.

Also it gives you not only the phase angle but also voltage difference in dB, thus it can give out vector values which can be transformed into rectangular values. Lotsa flexibility.

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I wanted to do something similar, I bought the pieces parts, but I haven't tried it.

So the idea is to get a sinewave and it's quadrature from a DDS. (my cheap (~$400) Rigol DG1022 seems to do OK up to ~10MHz... you have to push a button to sync the two phases) I would use one phase to stimulate my DUT. And put a TIA opamp circuit below that to measure the current. (Did I mention the differential voltage measurement across the DUT?) I was then going to use the two quadrature signals as references and do phase sensitive detection of the two signals (voltage and current)
(A fast comparator-> analog switches-> switched gain opamp->low pass filter)

In principle you then can measure both phases of the current.
I expect the phase shift will get 'interesting' at the higher frequencies.

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